The use of radio transmission between the surface and miners located deep underground using high frequencies has been considered as early as the 1920's in such works as "Underground Signaling with Radio Sets" by C. L. Colburn, C. M. Bouton, and H. B. Freeman, U.S. Bureau of Mines, R. I. 2407, 1922; "Radio as a Method of Underground Communication in Mines" by J. J. Jakosky, U.S. Bureau of Mines, R. I. 2599, 1924; "Factors Retarding Transmission of Radio Signals Underground, and Some Further Experiments and Conclusions" by J. J. Jakosky and D. N. Zellers, U.S. Bureau of Mines, R. I. 2561, 1924; and "Experiments in Underground Communication Through Earth Strata" by L. C. Ilsely, H. B. Freeman and D. N. Zellers, U.S. Bureau of Mines, T. P. 433, 1928.
In recent years, attention has shifted from the use of high frequencies to the use of low frequencies such as 300--5000 Hz. This work has been documented in "Radio Transmission to and from Underground Coal Mines, Theory and Measurement" by D. B. Lange, L. Ball and A. J. Farstad, I.E.E.E. Transaction on communication Vol. Com-21, March 1973.
Additionally, work has been undertaken relating to the use of prospecting by creating currents in subsurface ore deposits. This work is documented in the paper "AFMAG--Airborne and Ground" by S. H. Ward, Geophysics, Vol. XXIV, No. 4 (October 1959), pp. 761-789. This work recognizes that atmospheric noise would affect the signal generated by the ore deposits, and uses the atmospheric noise as a primary field. In the audio frequency range, this field penetrates deep enough into the soil to excite eddy currents in subsurface ore deposits of high conductivity. The resulting distortion of the electromagnetic field on the surface, indicated by the "strike angle" and the "dip angle", allows conclusions as to the size and depth of the deposit. However, this approach does not cancel the atmospheric noise, but merely utilizes it in determining the position and extent of the deposit.
U.S. Pat. No. 3,777,267 issued to Van der Floe et al discusses a method and apparatus for locating people who are buried underground. As described in this patent, the transmitter and receiver are contained in a single unit. The receiver stage contains an antenna and an input circuit selectively adjustable to at least two different levels by utilization of a transistor and an operational amplifier for receiving and amplifying the input signal. However, this device does not mention or take into account the presence of atmospheric noise in the area of search.
U.S. Pat. No. 3,508,238 issued to Buford M. Baker discloses an intrusion detection system utilizing a double loop. This double loop configuration utilizes the earth's magnetic field to detect entry into a given area.
None of these prior art references suggest the use of a double loop receiver which acts to cancel atmospherics so that the position of a subsurface transmitter can be ascertained.